Radioactive toxins

Choose your poison

Polonium-210 was efficient in its deadly way but needed experts to make it

THE substance that killed Alexander Litvinenko, polonium-210, belongs to a group of radioactive poisons that can be used to kill while leaving the murderer unscathed. The assassin would have been able to carry the toxin in a glass bottle knowing that its radioactivity would be readily absorbed by the vial itself. It cannot pass through skin. Opening the bottle at arm's length would be enough to protect the killer from receiving a deadly dose. Only when the substance is inhaled or swallowed does it become fatal.

This is because the radioactivity emanating from polonium-210 is in the form of highly energetic alpha particles. These interact strongly with matter and so can travel just a centimetre or so in air before being halted in their tracks. Once ingested, the particles dump huge quantities of energy in the body.

Polonium-210 is particularly deadly because it travels easily through the body. Once it has got into the blood, it is distributed so widely that it becomes a whole-body dose of radiation. It kills organs, tissues and cells or else damages them to the extent that they can no longer function. The poisoning is relatively rapid and a millionth of a gram is sufficient to kill. Mr Litvinenko probably received many times this dose.

The isotope does occur in nature but only extremely rarely. However, nuclear reactors generate 100 grams of the substance worldwide each year. It can be bought: it is used in industry to eliminate static electricity generated by such processes as paper rolling, the manufacture of sheet plastics and the spinning of synthetic fibres. But the radioactive material is sealed in small beads as a safety measure before it is sold. These beads could not be inhaled and, if they were swallowed, would pass through the body relatively safely.

A particle accelerator, a machine used by physicists to explore the fundamental building blocks of nature, could have been modified to produce polonium-210, though it would be an inefficient way of making the substance. In whatever way it was done, the poisoning appears to be the work of someone with access to well-equipped scientists.

Mr Litvinenko is thought to be the first person to die from acute polonium-210 poisoning. However, the first chronic case took place half a century ago. Ironically it was Irène Joliot-Curie, the daughter of Marie and Pierre Curie who first isolated polonium in 1898, who was the first to die because of exposure to it. A sealed capsule of polonium exploded in her laboratory and her death from leukaemia a decade later, in 1956, was attributed to the accident.